1. Field of the Invention
The present invention relates generally to an electrowinning cell for recovering copper from an electrolyte solution and, more particularly, to a manifold system for use with such an electrowinning cell operable to improve the circulation of electrolyte between the various electrodes of the cell and minimize the power requirements of both the cell and it's associated manifold system circulation pump.
2. Description of the Prior Art
The electrowinning of copper is becoming increasingly important to the competitiveness of the domestic copper industry. Production of electrowon copper has increased steadily since 1985, comprising 28% of the total domestic copper production, or 449,000 tons, in 1991. The energy requirement for producing copper in the electrowinning process is estimated to be 7.9 MJ/Kg (1 kw-hr/lb), which accounts for 20% of the energy requirement for producing copper in the leaching-solvent extraction-electrowinning (L-SX-EW) process. Given that the cost of energy will increase in the future, successful efforts to decrease the energy requirement for copper electrowinning will enhance the cost-effectiveness of the L-SX-EW process and will strengthen the competitiveness of the domestic copper industry.
One way to reduce the energy requirement for copper electrowinning is to use the ferrous/ferric anode reaction. The use of the ferrous/ferric anode reaction in copper electrowinning cells lowers the energy consumption of the cells as compared to conventional copper electrowinning cells which use the decomposition of water anode reaction. This is because the oxidation of ferrous to ferric iron occurs at a lower voltage than does the decomposition of water. However, maximum voltage reduction (and thus energy reduction) does not occur using the ferrous/ferric anode reaction unless effective circulation of electrolyte is achieved between the electrodes of the cell. This is due to the fact that the oxidation of ferrous to ferric iron in a copper electrolyte is a diffusion controlled reaction.
Several different schemes have hereto been employed in an attempt to improve the circulation of electrolyte between the electrodes of electrowinning and electrorefining cells without changing the design of the cells or electrodes. These known schemes include bubbling air from the bottom of the cell up between the electrodes, using sonic energy to induce circulation, and injecting electrolyte into the spaces between the electrodes using an electrolyte circulation manifold.
Air bubbling and sonic energy have associated with them environmental problems that affect the safety of the workers at the electrochemical facility. The electrowinning and electrorefining cells are open baths with the electrodes immersed into the electrolyte from the top of the cells. Air bubbles injected from the bottom of a cell rise and burst as they reach the top of the cell. The combination of many bursting bubbles causes a fine mist of the electrolyte to be carried up into the air above the cells. A principle component of the electrolyte is sulfuric acid. The electrolyte misting that occurs as a result of air bubbling increases the exposure of workers at the facility to sulfuric acid, affecting the worker's eyes and lungs. The use of sonic energy is also not preferred since sonic energy would affect workers' ears.
It is generally recognized that injecting electrolyte into the spaces between the electrodes of the electrowinning cell using a circulation manifold is the most effective way to induce electrolyte circulation without changing the design of the electrowinning and electrorefining cells and electrodes. In addition, this approach does not threaten the health of facility workers as does the utilization of air bubbling and sonic energy.
Although the use of a circulation manifold in place of air bubbling and sonic energy has been suggested and investigated to a certain extent, electrolyte circulation manifolds that circulate electrolyte over the entire face of the individual electrodes in the cell are currently viewed as requiring too much pumping energy to be useful.
Examples of various circulation manifold designs are disclosed in a paper entitled "The Electrowinning Of Copper Utilizing SO.sub.2 And Graphite Anodes", J. C. Stauter and G. F. Pace, 75th Annual General Meeting Of The Canadian Institute Of Mining And Metallurgy, Vancouver, B.C. Canada, Apr. 15-18, 1973, and in U.S. Pat. No 3,876,516 to Pace et al. These disclosed circulation manifold designs are utilized in small-scale cells with electrodes 3 inches wide and 4 inches tall. The manifold itself includes at least three 1/64 inch diameter electrolyte injection holes located at each space between adjacent electrodes. One hole injects electrolyte vertically up between the adjacent electrodes and one hole on each side of the "vertical" hole injects electrolyte at 30 degrees from the vertical. However, it is noted that when the circulation manifold was scaled up for use with industrial size electrodes approximately 34 inches wide and 48 inches tall, a total of eight injection holes, four holes at each side of the cell and directed at each other, were used at each space between adjacent electrodes. This design significantly increases the complexity of the design itself and provides a clear indication of the difficulty of circulating electrolyte between the electrodes of a full-scale electrowinning cell.
Another known circulation manifold design for use in full-scale cells combines electrolyte injection and suction. In this design scheme, at each space between adjacent electrodes one 1/4 inch injection hole injects electrolyte vertically from the bottom center of the cell while two 3/8 inch suction holes, one on each side of the injection hole, remove electrolyte from the cell. Still another known circulation manifold design for use in full-scale cells includes one 1/4 inch hole injecting electrolyte at a 45 degree angle from the bottom corner of the electrowinning cell at each space between adjacent electrodes. This manifold is designed to circulate electrolyte over the bottom one-third of each cathode in the cell and relies on the fact that in conventional copper electrowinning, decomposition of water to form oxygen is the reaction occurring at each anode in the cell, producing bubbles that rise to the surface of the cell. This manifold design relies on the oxygen bubbling at each anode to provide electrolyte circulation for the top two-thirds of each cathode. However, oxygen bubbling produces acid misting which is hazardous to facility workers. In order to mitigate the misting problem, plastic balls or a foam layer are placed on the top of the cell in an attempt to cover the open cell top.
As may be seen from the foregoing, although several different circulation manifold designs exist for use in a copper electrowinning cell to circulate electrolyte through the cell, none of the known designs are without their difficulties. The difficulties include high energy consumption and/or the inability to circulate over the entire face of the electrodes without acid misting present. Consequently, there is a need for an improved electrolyte circulation manifold design for use with a copper electrowinning cell which both reduces the power requirements of the cell itself and its associated manifold circulation pump, and also enhances the safety aspects associated with the operation of the electrowinning cell.